The basics of television are well known and described in numerous publications. For example, Television Electronics: Theory and Service, by Kiver and Kaufman (1983), which is herein incorporated by reference, describes, among other things, the basics of conventional analog television (see Chapters 7-10, pgs. 159-271). Additionally, the Television Engineering Handbook by Benson and Whitaker (1992), which is also herein incorporated by reference, describes digital television as well as High Definition Television (HDTV) (see Chapters 18 and 24, respectively).
Digital television, in view of its many advantages, is making great progress in its attempts to replace conventional analog television. This progress is being driven by many groups and associations. One such association of companies driving digital television is known as the "Digital HDTV Grand Alliance" including members such as AT&T, Philips, David Sarnoff Research Center, Massachusetts Institute of Technology and others.
An overview of the strides made by this group are presented in an article by Robert Hopkins entitled "Digital Terrestrial HDTV for North America: The Grand Alliance HDTV System" published in the IEEE Transactions on Consumer Electronics (Summer 1994) (hereinafter "the Grand Alliance article"). This article is also herein incorporated by reference for all of its teachings regarding the background and basics of digital HDTV systems including the use of Program and Transport Packet Streams.
Although the present invention is generally applicable to digital television and not just digital HDTV, the Grand Alliance article provides background material adequate to illustrate conventional systems and the problem associated therewith. In the Grand Alliance article, FIG. 1, also reproduced herein as FIG. 5, shows a high-level functional block diagram of the Grand Alliance HDTV System encoder. As shown, a video source delivers a video signal to the video compressor where video compression is accomplished in accordance with MPEG-2 Video standard at the Main Profile/High Level. It should be noted that the same problems may exist with a High Profile/Main Level system as well.
The video encoder output is packetized in variable-length parckets of data called Packetized Elementary Stream (PES) packets. Similar processing occurs for the audio input. The video and audio PES packets, along with any ancillary data are presented to a multiplexer (also known as a Transport Mutliplexer). The output of the multiplexer is a stream of fixed-length 188-byte MPEG-2 Transport Stream packets. The MPEG-2 Transport Stream packets are presented to the modulator such as a VSB or QAM modulator where data is encoded for the channel and a modulated carrier is generated. The modulated carrier occupies a typical MPEG transmission spectrum as shown in FIG. 2.
After transmission, the signal is received and decoded and displayed on whatever channel the user has selected. In the above-described configuration, a serious drawback exists, however, such that when the user decides to change channels (also known as "channel surfing") a significant delay may occur between the time the user selects a new channel and when the digital HDTV signal for that channel is actually acquired and displayed on the screen. This delay can be as much as 1 to 2 seconds in duration and, for even the most novice channel surfers, quite annoying. This delay occurs due to the fairly complex decoding schemes implemented in a digital television receiver, during a channel change, before a quality picture can be displayed. Accordingly, it is desirable to fill the delay time with a adequate picture.